1. Field of the Invention
The present invention relates to a light emitting diode (hereinafter referred to as “LED”), and a fabrication process therefor. More particularly, the present invention relates to a high-density mounting LED suitable for use in large area light emitting device display and illuminating facilities and a fabrication process therefor. Also, the present invention relates to a light-emitting device using the LED with a superior heat sink property and a fabrication process therefor.
2. Description of the Prior Art
LED is one kind of solid lighting indicators. LEDs for single colors including RGB (red, green and blue) colors, three primary colors of light, have been followed by LEDs for W (white) colors, which have utilized in more various fields. Recently, LEDs have been developed from lamp types to surface mount device (SMD) types that allow LEDs to be mounted at so high a density as to make a large area display. According to such a tendency, LEDs are used in a broader applications, from indicators to back light sources for LED displays, further to a next-generation lighting system which is substitutive for conventional lighting system such as incandescent electric lamps, fluorescent lamps or street lamps. Unlike general lamps, lighting systems using LEDs have simple lighting circuits and need not invert circuits nor core type ballasts. Over incandescent lamps, LED luminaire has the advantage of being lower in maintenance cost because of its being operated at less electrical power, with 10-times longer lifetime.
Representative example of white LED applicable to luminaire is disclosed in Japanese Pat. Laid-Open Publication No. 2000-315826 which describes a light emitting device composed of an LED and a phosphor. The light emitting device of this patent, as shown in FIG. 1a, comprises a blue LED chip 3 mounted on a ceramic substrate 1, a first transparent coating 6 covering the blue LED chip 3, a second transparent coating 6a positioned on the first transparent coating 6, and an electrode electrically connected to the LED chip 3 via a wire 5, the second transparent coating 6a containing a phosphorescent material. In this structure, the fluorescent material absorbs the visible light beam emerging from the LED chip to radiate fluorescent light so that mixing the visible light of the LED chip with the fluorescent light emitted from the fluorescent material results in emission of white light. This light emitting device can emit homogenous white light with excellent efficiency. Another LED which is able to emit white light may be found in relevant literature (e.g., U.S. Pat. Nos. 5,998,925 and 6,069,440).
Generally, appliances with these LEDs are known deteriorate in light emitting properties mainly owing to thermal stress. In a lighting system or a traffic signal, which is fabricated by mounting a multitude of conventional LED chips on a substrate at a high density, as shown in FIG. 1b, LEDs suffer from more overheating than LEDs in any other applications and have a tendency to radiate heat in proportion to the total lighting area. Particularly in the case of blue LEDs, the heating problem is more serious because they are operated at higher voltages compared to LEDs of other colors. Further, the larger areas of LED luminaire aggravate the performance and out-of-order rate of the LEDs used. This trend is also found when LEDs are mounted at higher densities in a given area. Conventional light emitting devices, most of which are structured as illustrated in FIG. 1b, are so poor in heat sink capability as to show a limit in mounting LED chips with a high density in a large area.